Wavelength division multiplexing (WDM) has been explored as an approach for increasing the capacity of fiber optic networks. In a WDM system, plural optical signals or channels are carried over a single optical fiber with each channel being assigned a particular wavelength. Such systems typically include a plurality of receivers, each detecting a respective channel by effectively filtering out the remaining channels.
Optical signals or channels in a WDM system are frequently transmitted over silica based optical fibers, which typically have relatively low loss at wavelengths within a range of 1525 nm to 1580 nm. WDM optical signal channels at wavelengths within this low loss “window” can be transmitted over distances of approximately 50–100 km without significant attenuation. For distances beyond 100 km, however, optical amplifiers are required to compensate for optical fiber loss.
Optical amplifiers have been developed which include an optical fiber doped with erbium. The erbium-doped fiber is “pumped” with light at a selected wavelength, e.g., 980 nm, to provide amplification or gain at wavelengths within the low loss window of the optical fiber. However, erbium doped fiber amplifiers do not uniformly amplify light within the spectral region of 1525 to 1580 nm. For example, an optical channel at a wavelength of 1540 nm, for example, is typically amplified 4 dB more than an optical channel at a wavelength of 1555 nm. While such a large variation in gain can be tolerated for a system with only one optical amplifier, it cannot be tolerated for a system with plural optical amplifiers or numerous, narrowly-spaced optical channels. In which case, much of the pump power supplies energy for amplifying light at the high gain wavelengths rather than amplifying the low gain wavelengths. As a result, low gain wavelengths suffer excessive noise accumulation after propagating through several amplifiers.
Accordingly, optical amplifiers providing substantially uniform spectral gain have been developed. In particular, optical amplifiers including an optical filter provided between first and second stages of erbium doped fiber are known to provide gain flatness. In these amplifiers, the first stage is operated in a high gain mode and supplies a low noise signal to the second stage, while the second stage is operated in a high power mode. Although the second stage introduces more noise than the first, the overall noise output by the amplifier is low due to the low noise signal of the first stage. The optical filter selectively attenuates the high gain wavelengths, while passing the low gain wavelengths, so that the gain is substantially equal for each wavelength output from the second stage.
These gain-flattening amplifiers are typically designed to receive optical signals at a particular power level. In the event the total power level of all optical signals input to the amplifier differs from the desired input level, the amplifier can no longer amplify each wavelength with substantially the same amount of gain. Accordingly, the conventional gain-flattened amplifiers discussed above are unable to receive input optical signals over a wide range of power levels while maintaining substantially uniform gain at each wavelength.
U.S. Pat. No. 6,057,959, incorporated by reference herein, discloses use of a variable optical attenuator provided between first and second stages of an optical amplifier to offset deviations in optical input power away from an optimal input power for which the amplifier is designed. Without the variable optical attenuator, the amplifier can suffer from “tilt”, in which amplifier output power increase or decreases from one optical signal to the next such that power spectrum of the WDM signal has a uniform slope. By appropriately adjusting the variable optical attenuator, a substantially uniform spectral output can be achieved, or if desired a predetermined tilt can be achieved.
In so-called ultra-long haul WDM systems, relatively large numbers of optical amplifiers are provided between transmitters and receivers. Often twenty concatenated optical amplifiers are provided, spaced 50–100 km apart, to extend propagation distances 1000–3000 km. In such systems, however, a “ripple” phenomenon can occur in which slight power variations among the WDM signals are amplified as the signals pass through successive amplifiers. These power variations can stem from an unequal loss spectrum caused by badly mated connectors and tight fiber bends. At the receive end, the ripple can be relatively large such that low gain wavelengths can incur excessive noise accumulation. Conventional techniques discussed above are often ineffective in eliminating ripple.
Moreover, numerous transmission, as well as dispersion compensating, optical fibers, are currently available, each having its own loss spectrum. Accordingly, it is difficult to design an optical amplifier so that it will have a uniform output spectrum for every fiber type.